The microclimate of the drainage differs markedly from that of other stations. The major difference is attributable to the topography of the drainage itself. Nocturnal cold air flows from the surrounding mesa top to lower elevations. A lake of cold air forms in the bottom of the drainage; the depth of the lake depends in part upon the depth of the drainage. The same phenomenon occurs in canyons and causes cooler night time temperatures on the floor of canyons than on adjacent mesa tops (Erdman, Douglas, and Marr, in press). Drainage of cold air into lower elevations affects both nocturnal air temperatures and relative humidities. Table 4 shows that maximum air temperatures in the drainage did not differ appreciably from those at other stations. Mean minimum temperatures, however, were considerably lower in the drainage than at the other sites. This phenomenon is reflected also in the mean air temperatures at this station. [Illustration: FIG. 10: (above) Photograph of microclimatic station at the Far View Sagebrush Site, at trapping station F4a in the grid south of Far View Ruins. Dominant vegetation is _Artemisia tridentata_.] [Illustration: FIG. 11: (below) Photograph of microclimatic station at the Drainage Site, in the bottom of a shallow drainage at trapping station M4d of the grid south of Far View Ruins.] The drainage site had the highest humidities of all stations each month in which data were collected (Table 4). Relative humidities of 90 to 100 per cent were common in the drainage, but occurred at other stations only in rainy periods. For example, in the month of August, 26 of the daily maximum readings were between 95 and 100 per cent at the drainage site, but at the other stations relative humidities were above 95 per cent for an average of only nine nights. Minimum humidities were about the same for all stations, since they are affected by insolation received during the day, and not by the drainage of cold air at night. _Oak Brush Site, 7,640 feet elevation_ The station was in an oak thicket at trapping station L4a, 250 feet south and 50 feet east of the drainage site on a southwest-facing slope of about 30 degrees (Fig. 12). The station was on the lower third of the slope, approximately 15 feet higher than M4d, the station in the bottom of the drainage. _P. truei_ and _P. maniculatus_ occur together in this area. Air temperatures and relative humidities at this station did not differ appreciably from mean temperatures and humidities at the other stations. The unusual feature is the lack of evidence of cold air drainage. The lake of cold air in the bottom of the drainage apparently is too shallow to reach this station. This site is near the head of the drainage, and the cold, nocturnal air probably moves rapidly down slope into the deeper parts of the canyon, rather than piling up at the shallow head of the drainage.